Particulate catalysts are commonly employed in chemical, petroleum and petrochemical processing for chemically altering, or converting various gas and liquid feeds to more desirable products. Such catalysts are formed by dispersing a catalytically active metal, or metals, or the compounds thereof upon particulate carriers, or supports. In accordance with the conventional wisdom the catalytically active metal, or metals is generally dispersed as uniformly as possible throughout the particle, providing a uniformity of the catalytically active sites from the center of a particle outwardly.
Fisher-Tropsch synthesis, a process for the production of hydrocarbons from carbon monoxide and hydrogen (synthesis gas), has been commercially practiced in some parts of the world. This process may gain wider acceptance, perhaps in this country, if adequate improvements can be made in the existing technology. The earlier Fischer-Tropsch catalysts were constituted for the most part of non-noble metals dispersed throughout a porous inorganic oxide support. The Group VIII non-noble metals, iron, cobalt, and nickel have been widely used in Fischer-Tropsch reactions, and these metals have been promoted with various other metals, and supported in various ways on various substrates, principally alumina. Most commercial experience, however, has been based on cobalt and iron catalysts. The first commercial Fischer-Tropsch operation utilized a cobalt catalyst, though later more active iron catalysts were also commercialized. The cobalt and iron catalysts were formed by compositing the metal throughout an inorganic oxide support. The early cobalt catalysts, however, were of generally low activity necessitating the use of a multiple staged process, as well as low syn gas throughput. The iron catalysts, on the other hand, produce too much carbon dioxide from the synthesis gas with too little of the carbon monoxide being converted to hydrocarbons.
Cobalt catalysts of improved activity have nonetheless been made. For example, U.S. Pat. No. 4,542,122 by Payne et al, which issued Sept. 17, 1985, describes improved cobalt catalyst compositions useful for the preparation of liquid hydrocarbons from synthesis gas. These catalyst compositions are characterized, in particular, as cobalt-titania or thoria promoted cobalt-titania, wherein cobalt, or cobalt and thoria, is uniformly dispersed throughout the titania, or titania-containing support particles, especially a high rutile content titania. U.S. Pat. No. 4,568,663 by Mauldin, which issued Feb. 4, 1986, also discloses cobalt-titania catalysts to which rhenium is added to improve catalyst activity, and regeneration stability. In these catalysts both the cobalt and rhenium are uniformly dispersed throughout the support particles. In pending application Ser. No. 072,517, supra, there is described a cobalt catalyst, or catalyst wherein cobalt and an additional metal, or metals, e.g., rhenium, hafnium, zirconium, cerium, thorium, uranium or the like is dispersed as a thin film within a critical range of thicknesses on the peripheral, or outside surface of a particulate support, notably a titania or titania-containing support. These catalysts achieve high productivity, at low methane selectivity, in Fischer-Tropsch reactions. Productivity, which is defined as the standard volumes of carbon monoxide converted/volume catalyst/hour, is, of course, the life blood of a commercial operation. In the use of these catalysts, productivity in a Fischer-Tropsch reaction is considerably increased as contrasted with catalysts otherwise similar, employed at similar conditions, except that a similar metal, or metals, is uniformly dispersed throughout the particulate support. In application Ser. No. 072,517, data are given, and claim is made to forming catalysts by dispersing thin catalytically active surface layers of cobalt on heated support particles by repetitively spraying the particles with a solution of the cobalt compound.